1. Field of the invention
The present invention relates to a power transfer device for use in a four-wheel drive vehicle, more particularly to a power transfer device capable of controlling the split ratio of the drive torque to the front and rear road wheels in accordance with driving conditions of the vehicle.
2. Discussion of the background
In conventional power transfer devices for four-wheel drive, a differential of the bevel gear type or the planetary gear type has been adapted to transfer drive torque from a prime mover to the front and rear road wheels at a constant gear ratio. It is, however, noted that the steering stability of the vehicle is in a close relationship with the split drive torque applied to the front and rear road wheels. For this reason, it is desirable that the split ratio of the drive torque to the front and rear road wheels is controlled in accordance with road conditions and driving conditions such as frictional coefficient of the road surface, inclination of the road, acceleration of the vehicle, cornering travel of the vehicle and the like. For such control of the split ratio of the drive torque, the Japanese Patent Early Publication No. 59 - 151661 discloses a power transfer device wherein a continuously variable transmission is disposed in a front or rear wheel drive power train of the differential. In this power transfer device, the change-speed ratio of the transmission is changed in accordance with driving conditions of the vehicle to control the split ratio of the drive torque to the front and rear road wheels. It is, however, disadvantageous that the power transfer device becomes large in size and heavy due to provision of the continuously variable transmission.